In multi-carrier communications, data may be transmitted over a local twisted-pair telephone line between a central office (“CO”) and a residential or business location, commonly referred to as a remote transmitter (“RT”). Data, typically starting as binary information is modulated into a signal, which is sent over a twisted-pair telephone line to a receiver located at the RT. The receiver preferably obtains and samples the signal, whereupon the signal is transformed back into binary information (e.g., a discrete-time signal). At the receiver, accurate timing recovery, commonly consisting of sampling frequency and symbol alignment, is important in order for the receiver to properly extract meaningful binary information from the signal.
Digital Subscriber Line (“DSL”) is a transmission technology typically used in multicarrier communications for bringing high bandwidth information to homes and small businesses over ordinary copper telephone lines. Different variations of DSL often referred to as xDSL include asymmetric DSL (“ADSL”), DSL Lite, high bit rate DSL (“HDSL”), rate adaptive DSL (“RADSL”), and very high data rate DSL (“VDSL”). DSL and its variation xDSL preferably provide a customer located at the RT with multicarrier or single carrier technology often tailored for different applications.
Standards have been created for providing accurate timing recovery systems for DSL and xDSL communication systems. For example, in an effort to provide an accurate timing recovery system for asymmetric digital subscriber line (“ADSL”) transceivers, the American National Standards Institute (“ANSI”) put forth Standard T1.413, the contents of which are incorporated herein by reference, to include standards for timing recovery systems. The ADSL standard specifies a dedicated pilot tone for use in the timing recovery system. The pilot tone, typically embedded in a transmitted signal in ADSL communications, is analyzed to recover timing information used to provide sample frequency and symbol alignment, typically using a variety of available software and hardware techniques.
A disadvantage of a dedicated pilot tone is that typically a carrier is deactivated in order to carry a pilot tone instead of carrying data. By doing this, systems such as narrow band communication systems (e.g., a system having spectral content less than typically found in a broadband system) may waste between 5-10% of the throughput.
Other disadvantages of a dedicated pilot tone may introduce frailties into multicarrier communication systems such as through the use of an unmodulated carrier and perhaps by producing a higher variance of timing error. The single unmodulated carrier used to transmit the pilot tone ordinarily represents a fraction of the received signal power possibly introducing a higher variance of timing error. Furthermore, since the pilot tone is typically sent using an unmodulated carrier (i.e., unmodulated pilot tone signal) temperature, interference, and noise may have more influence on the signal possibly corrupting the pilot signal resulting in an undesirable sampling frequency at the receiver, perhaps leading to lost data and ultimately to a catastrophic receiver failure.
Thus, there is a need to provide robust timing recovery while preferably maintaining available throughput in multicarrier communication systems.